(1) Field of the Invention
This invention relates to a mask and method of forming a mask for forming closely spaced and larger isolated contact holes in integrated circuit wafers. More specifically the invention relates to a combined binary and ring type attenuating phase shifting mask.
(2) Description of the Related Art
Attenuating phase shifting masks are frequently used to form contact holes in integrated circuit wafers. These masks often have problems due to side lobe effect due to the non zero light transmission of the attenuating phase shifting material.
The book "Handbook of Microlithography, Micromachining, and Microfabrication, Volume I: Microlithography, edited by P. Rai-Choudhury, SPIE Press, 1977, pages 74-82 discusses deferent types of phase shifting masks including attenuating phase shifting masks.
U.S. Pat. No. 5,700,602 to Dao et al. describes a method and apparatus for determining the actual phase shift for a reticle having a phase shifted feature.
U.S. Pat. No. 5,716,738 to Garza describes a mask using attenuating phase shifting material and a light opaque border. The light opaque border is preferably photosensitive polyimide.
U.S. Pat. No. 5,429,897 to Yoshioka et al. describes an attenuating phase shifting mask which has an attenuating type phase shifting pattern and an auxiliary attenuating type phase shifting pattern.
U.S. Pat. No. 5,288,569 to Lin describes feature biassing in phase shifting masks to improve the exposure latitude and depth of focus.
U.S. Pat. No. 5,474,865 to Vasudev describes a globally planarized binary optical mask having absorbers buried in the mask substrate.
U.S. Pat. No. 5,477,058 to Sato describes an attenuating phase shifting mask having a circuit pattern formed of attenuating phase shifting material. The reticle alignment marks are made of a separate substantially opaque layer.
U.S. Pat. No. 5,527,645 to Pati et al. describes a systematic method for producing phase shifting masks.